Polyscias oahuensis is a tree species that reaches heights between 3 to 10 meters. It exhibits a robust growth form characterized by compound leaves, which are alternately arranged. The blades span 15–35 cm long, comprising 7 to 15 leaflets. These leaflets range from broadly elliptic or ovate to narrowly oblong-elliptic, measuring 5–13 cm in length and 2.5–8 cm in width, with the basal pair typically smaller. The apices of the leaflets are predominantly rounded to acute, seldom acuminate, transitioning to a base that is cuneate to rounded or truncate, occasionally subcordate, and often oblique, all while maintaining a glabrous surface. The entire margins and petiolulate attachment, with petiolules measuring 5–18 mm, contribute to the leaf’s structure, while stipules are either absent or rudimentary. The flowers are organized into erect, compound-umbellate, or occasionally racemose-umbellate inflorescences 2–3 times. The primary axis of the inflorescence is either very short or absent, with secondary axes extending 7–15(–18) cm, each terminating in an umbellule of flowers. They are bisexual, actinomorphic, and supported by 6–18 mm long pedicels, which may elongate up to 22 mm in fruit. The calyx is subtly defined, while the corolla comprises 5–7(8) petals in shades of pinkish or yellowish green. The stamen count ranges from 5 to 25(–30), encircling an ovary inferior to ¾ inferior and capped by a nectary disk. The fruits are blackish-purple drupes, which are either globose or ovoid to subcylindrical and often exhibit strong ribbing when dry, measuring 7–15 mm in length. Each drupe encases a single seed, containing a small embryo. It occurs in mesic to wet forests between 240-1,520 meters.